Wave filter



Jan. 14,1947. w.'P. MASON 4,

' WAVE FILTER Filed Aug. 19. 1943 FIG 4 TTENUA 7/0 8 5; J 2 rm :2

FRE QUE/VG Y INVENTOR I I I By may/150M @4 4 KM A TTORNEV Patented Jan. 14, 1947 WAVE FILTER- l Warren P. Mason,

West Orange, N. .L, assignorto Bell Telephone Laboratories, Incorporated, New York, N Y., a. corporation of New York Application August 19, 1943, Serial No. 499,222

Claims. (c1.17s 44) This invention relates to wave filters and more particularly to those employing sections of transmission line as component impedance elements. The principal object of the invention is tosuppress one or more bands of high frequencies while freely transmitting other frequencies. Other objects are to decrease the attenuation distortion, sharpen the cut-olfs, increase the height of the attenuation peaks and increase the power carrying capacity of wave filters intended for use at high frequencies.

The filter in accordance with the invention comprises sections of transmission line and .capacitors arranged in the form. of a bridged-T structure having two equal series arms, an inter-= posed shunt branch and a bridging branch. The serie arms are constituted by two sections of transmission line connected in tandem. The shunt branch may also include one or more sections of line and one or more capacitors. If a band-suppression characteristic is desired, the bridging branch includes a capacitor. sections are preferably of the'coaxial type. The sections used in the series arms are preferably arranged physically in parallel, with their ends close together, so that only short connections will be required .for the bridging branch. Certain of. the capacitors may be built within the line sections to ensure adequate shielding and short connections. By proper design of the line sections the dissipation therein may be kept low,

having small distortion within the transmission. band, sharp cut-oils and high resulting in a filter attenuation peaks. If sufficient separation is provided between inner and outer conductors; a

The line arated ifrornthe plate I! suppression filter in accordance with the invention connected to a coaxial transmission line I having a cylindrical'outer conductor 2 and an inner conductor 3. The filter comprises two similar tandem-connected sections of transmission line 4 and 5, branching off from the line I, and a third section of line 6 connected at the junction of the sections 4 and 5. In order to save drawing space, portions of the sections 4 and 5 have been out out. As shown, each line section is of the coaxial type, comprising a hollow cylindrical outer conductor and, concentric therewith, a cylindrical inner conductor. The sections 4, 5 and 6 have outer conductors 1, 8 and 9, respectively, and inner conductors III, II and 12, respectively. The cylinder 9 is closed at its upper end by a circular metallic end plate I3. The cylinders 1 and 8 are arranged side by side, physically in parallel, and extend upward from the plate. 13 to their points of connection with the line I. The inner conductors I 0 and H extend through the apertures I4 and 15,. respectively, in. the plate 13 and terminate in the circular metallic condenser plate 11. The plate I3 is sepby a disk I9 of dielectric material which also hasholes to permit passage therethrough of the conductors, l0 and-ll.

large amount of power may be transmitted without voltage breakdown.

.The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing in which like reference characters refer to similar or corresponding parts and in which: i

Fig. 1 is a longitudinal cross-section of a bandsuppression wave filter in accordance with the invention; 1

Fig. 2 is a simplified equivalent 5 electrical circuit used in designing the filter;

Fig. 3 gives a, typical attenuation-frequency characteristic obtainable with the filter;

Fig. 4 is another equivalent circuit used in one 4 step of the design; and

Fig. 5 is a more exact equivalent electrical circuit for the-filter of Fig. 1.

Taking up the figures in more detail, Fig. 1 is a longitudinal cross-sectional view of a band- Theinner conductor l2 terminates at its upper end in a circular metallic condenser plate 20 which is separated from the plate I! by a disk H of dielectric material. Extending downward from the plate 20 is a cylindrical condenser plate 22 separated from the outer conductor 9 by a cylinder 23 of dielectric material. The line section 6 is short-circuited at its lower end by means of the annular metallic member 24 which may be slid in or out to adjust the electrical length of the section. The portion of the inner conductor 3 between the points of connection of the conductors l0 and l is removed and a capacitor of capacitance C1, located within the line I, is connected between the upper ends of the conductors l0 andll; Input energy maybe applied at one end of the line at the points 25 and Hand the output taken off at the points 21 and 28.

"Fig.1?! shows asimplified equivalent electrical c'ircuit'for the is of the bridged-T type, comprising two equal seriesarms, an interposed shunt branch and a bridging branch. Each of the series arms is an inductance L1, the bridging branch'is the capacitance C1 and the shunt branchis constituted by a capacitance Cg connected in parallel with an arm-comprising an inductance L; in series with a capacitance C3. The filter is-of the band-supstructure of Fig. 1. The network pression type, having a lower cut-off at the frequency iii, an upper cut-oil at in and a peak of attenuation at an intermediate frequency f.., For maximum attenuation fa, should be the geometric mean of is and fa. A typical attenuation-frequency characteristic, assuming negligible dissipation in the component elements, is shown in Fig. 3.

The'design formulas for a filter of the type shown in Fig. 2 are given on page 284 of applicants book entitled Electromechanical Transducers and Wave Filters, published by D. Van. Nostrand Company, Incorporated. Using the notation of Fig. 2, the component elements have.

the following values:

L 47TZol:f f farads b oo 02 W farads (2) C3 TrZobfifZ 3: 2 f 2 farads (3) l henries (4) L: 4 ,1, foo z 2 (who? henries (5) in which i 2 -2 2 2 2 7gzifiz 4 (e and Z0 is the image impedance of the filter at zero frequency.

In some cases the inductance L2 will be too large to be furnished conveniently by a section of line. This difiiculty may be overcome, however, by adding a redundant capacitance to the shunt branch. First, the capacitance C2 is split into two capacitances C21 and C22, the sum of which is equal to C2. Then the portion of the shunt branch comprising L2, C3 and C22 is transformed into-the equivalent form of a capacitance C; connected in series with an anti-resonant loop made up of a capacitance C and an inductance L3. The formulas to be used for this transformation are the following;-

The resulting network is shown in Fig. 4.

It remains now only to explain how the mechanical structure of Fig. 1 may be designed to be electrically equivalent to the circuit shown in Fig. 4. An equivalent electrical circuit for the filter of Fig. 1 is shown in Fig. 5. Since each of the. line sections 4, 5 and 6 will ordinarily have a length which is only a small fraction of a wavelength at the frequency f m, it may be represented by an inductance having at each end a shunt capacitance. Each of the. sections 4 and 5 is represented by an inductance L1 and the two equal shunt capacitancesv CA, CA and the portion of, the section 6 of length B by the. inductance La and the equal shuntcapacitances CB, CE. The

approximate values of these elements are given by the following formulas:

in which A is the length in centimeters of each of the line sections 4 and 5, a1 and in are, respectively, the inner diameter of the outer conductor 1 or 8 and the outer diameter of the inner conductor H] or II, B is the length in centimeters of the-line sections 6 from the lower end of the cylinder 22 to the short-circuiting member 24 and az and 222 are, respectively, the inner diameter of "the outer conductor 9 and the outer diameter of the inner conductor 12. The capacitance C1 is the one connected between the upper ends of the inner conductors i0 and II in Fig. 1', C4 is the capacitance between the plates l1' and 20, C6 is the capacitance between the plates 13 and I1 and C7 is the capacitance between the cylinder 22 and the outer conductor 9.

It will be noted that the capacitance CB at the right end of L3 is shorted out by the member 24. The shunt capacitances CA at the ends of the filter are usually small enough to be neglected. When this is done, the circuit of Fig. 5 is seen to have the same configuration as that shown in Fig. 4. The two circuits will be equivalent when 2C'A+CG:C21 (14) and A suggested procedure for designing the filter of Fig. 1 is as follows: First, the cut-off frequencies IA. and f1; and the frequency of peak attenuation J... are chosen. Then the values of the capacitances C1, C2 and Ca and the inductances, L1 and L2 are computed from Equations 1 to 5. Next, the. capacitance C2 is split into C21 and C22 and the values of the elements C4, C5 and L3 found from Equations 7, S and 9. Now, the

required length A of each of the line sections 4 and. 5 is found from EquationlZ, using any convenient ratio of 171/111. However, to provide a minimum longitudinal cross-sectional area for the sections I and 2 this ratio should'be 9.2. The diameter 121 of the outer conductor will, in any event, be made large enough so that the dissipation is kept within allowable limits and the separation between the.conductors such as I and H1 is sufficient to withstand, without breakdown, the voltage to be appliedto the. filter. Next, the value of each, of the capacitances, CA is found from Equation 10. The value of the capacitance Cs may now befound from Equation 14. The area of the plate; it, the spacing between the plates l3 and I1 and the dielectric constant of the separator "9" are proportioned to give the required capacitance C6. The spacing between the plates I"! and 29 and the dielectric constant of the separator 2!- are proportioned to provide the required capacitance C4. The capacitance C1 is furnished by any suitable type of capacitor,

Next, the required, partial length B of; the line section 6 is found from Equation 13. The ratio of 172/112 may be taken as 9.2, for the reason given above, or as any other appropriate value. The diameter 122 of the outer conductor 9 may conveniently be chosen as approximately 2221. The value of the capacitance CB is found from Equation 11 and the valu of C7 from Equation 15. The length D of the cylinder 22, the spacing between the cylinder 22 and the outer conductor 9 and the dielectric constant of the separator 23 are proportioned to provide the required capacitance 07. There will be a small amount of inductance associated with the cylinder 22 but if its length D is kept short enough this inductance may be neglected.

Although the embodiment disclosed herein is a band-suppression filter, it will be apparent to those skilled in the art that other types such, for example, as low-pass, band-pass or high-pass, falling within the scope of the invention, may be provided by suitably modifying the component impedance branches in accordance with wellknown filter theory.

What is claimed is:

1. A wave filter comprising three sections of coaxial transmission line, an end plate and a condenser plate, one of said line sections extending in one direction from said end plate, the other two of said line sections extending in the opposite direction from said end plate and said condenser plate being located Within said one line section near said end plate and electrically connected to the inner conductors of said other line sections.

2. A filter in accordance with claim 1 which includes a second condenser plate located within said one line section near said first-mentioned condenser plate and electrically connected to the inner conductor of said one line section.

3. A filter in accordance with claim 1 which includes a second condenser plate located within said one line section near said first-mentioned condenser plate and electrically connected to the inner conductor of said one line section and a cylindrical condenser plate located within said one line section and electrically connected to said second condenser plate.

4. A filter in accordance with claim 1 which includes a capacitor connected between the ends of said other two line sections remote from said end plate.

5. A filter in accordance with claim 1 in which said other two line sections form the series arms of a T network,

6. A filter in accordance with claim 1 in which said other two line sections form the series arms of a bridged-T network.

7. Awave filter comprising three sections of coaxial transmission line, an end plate common to all three of said sections, and a condenser plate located within one of said sections near said end plate and electrically connected to the inner conductors of the other two of said sections.

8. A filter in accordance with claim 7 which includes a second condenser plate located within said one line section near said first-mentioned condenser plate and electrically connected to the inner conductor of said one line section.

9. A filter in accordance with claim 7 which includes a second condenser plate located within said one line section near said first-mentioned condenser plate and electrically connected to the inner conductor of said one line section and a cylindrical condenser plate located Within said one line section and electrically connected to said second condenser plate.

10. A filter in accordance with claim 7 which includes a capacitor connected between the ends of said other two line sections remote from said end plate.

WARREN P. MASON. 

